The process of drilling an oil or gas well is a dangerous process which involves the risk of fire. The risk of fire is particularly well known, but in spite of the high level of risk, fires are relatively few and far between. Nevertheless, they occasionally do happen on or at the drilling rig. The risk of fire is increased during certain drilling circumstances. Specifically, the risk of fire can be increased markedly when the well penetrates a high pressure natural gas formation. In that event, the well may experience a surge in which the high pressure natural gas flows up the well borehole during drilling. For that reason, the normal, safe operating procedures require that the well be protected by filling the well with a heavy weight drilling fluid, normally known as drilling mud. The drilling fluid is placed in the well to maintain a column of fluid acting on the formation which overcomes the gas pressure. This prevents a surge of natural gas from flowing rapidly up the borehole and escaping at the surface. In addition to that protection, there is also a blow out preventer (BOP) which is installed at the surface, typically being installed beneath the floor of the drilling rig. The blow out preventer is an emergency device which clamps off flow through the borehole.
Occasionally, problems can still arise even though the foregoing protective procedures are fully practiced. For instance, it may be necessary to use an oil base drilling fluid as opposed to a water base drilling fluid. There is another circumstance in which the risk of fire is increased. One example of this involves horizontal drilling. A good example of horizontal drilling is found in the formation which is known as the Austin Chalk which is a formation having various producing strata at about 7000-9000 feet in depth stretching from approximately Laredo, Tex. to the Northeast and extending almost to the Red River. The Austin Chalk is a tight formation which can be produced from vertical wells provided the well passes through the formation and intercepts a formation fracture. Wells of this sort can produce from the tight formation, but the well life has been somewhat shortened because of difficulties in lateral migration through the formation. In a number of occasions, vertical wells into the Austin Chalk have been produced for the useful lives and when that production has been depleted, the wells have been reworked by redrilling the last few hundred feet of the well. Typically, this involves the use of a milling tool to cut a window in the casing in the well, the window being about 100-300 feet in length, and then reentering the well at that region with the appropriate equipment to drill from or through the window in a radius of curvature until the well is then horizontal or substantially so. This requires that the deviated well form a horizontal leg which is ideally centered in the Austin Chalk and is ideally positioned at the midpoint of the formation. The horizontal leg can be several hundred feet long. Indeed, this rework procedure has proven so popular that it has now been adapted for new wells into the Austin Chalk formation.
Assume for purposes of description that the horizontal portion is to be 1000 feet in length. This forms an open hole of approximately 1000 feet in length in the Austin Chalk formation which is a producing formation with a substantial formation pressure drive. In that event, the entire drilling process through the Austin Chalk involves drilling while producing. That is, during the process of extending the horizontal leg to 1000 feet, the Austin Chalk formation will produce at all points along this horizontal leg and thereby seriously increase the risk of drilling. As the partially completed horizontal leg extends through the Austin Chalk, more and more oil or gas or both is produced and comingles with the drilling fluid. Since the drilling fluid is circulated to the well head in the annular space of the borehole, the mud handling procedures at the surface are frought with danger of fire because the oil and gas produced during this time is brought to the surface. There is a risk of fire where the annular flow comes to the surface which is immediately under the drilling rig at the BOP. There is an additional chance of fire hazard in the equipment area where the mud is also handled by the choke equipment, the shale shaker or other mud handling equipment. This equipment is normally deployed at one region at the time of rigging up. Suffice it to say, the circulation of drilling mud through the Austin Chalk brings to the surface some oil or gas, and thereby increases in the risk of fire.